1. Field of the Invention
The present invention relates to a display device. More particularly, the present invention relates to a digital/analog (D/A) converter, a light emitting display device using the converter, and a display panel and a driving method thereof.
2. Description of the Related Art
In general, an organic light emitting diode (OLED) display is a display device that electrically excites a phosphorous organic compound to emit light, and it voltage- or current-programs N×M organic light emitting pixels to display images. An organic light emitting pixel (or diode) of the OLED display includes an anode, an organic thin film, and a cathode. The organic thin film has a multi-layer structure including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) for maintaining balance between electrons and holes to improve emitting efficiencies, and it further includes an electron injecting layer (EIL) and a hole injecting layer (HIL).
Methods for driving the organic light emitting pixels include a passive matrix method, and an active matrix method using thin film transistors (TFTs) or MOSFETs. The passive matrix method forms cathodes and anodes to cross with each other, and lines are selected to drive the organic light emitting pixels. The active matrix method couples a TFT and a capacitor to each indium tin oxide (ITO) pixel electrode, and an organic light emitting pixel is driven by a voltage maintained by capacitance of a capacitor. The active matrix method can be classified as a voltage programming method or a current programming method depending on forms of signals supplied for programming a voltage to a capacitor.
It is difficult for a conventional voltage-programming pixel circuit to obtain high gray scales because of deviations of threshold voltages (VTHs) of TFTs and/or mobilities of carriers of the TFTs caused by non-uniformity of a manufacturing process. However, if a current source can supply a substantially uniform current to pixel circuits over a whole data line, a pixel circuit of a current programming method generates uniform display characteristics even when a driving transistor in each pixel has non-uniform voltage-current characteristics.
A digital/analog (D/A) converter, which converts grayscale data into a grayscale current and applies it to a pixel circuit, is necessary in a display device of a current programming pixel, and the D/A converter should correct a gamma characteristic of the grayscale data in consideration of a characteristic of a display panel when converting the grayscale data into the grayscale current.
However, although the gamma characteristic of the display panel is nonlinear with respect to the grayscale data, a conventional D/A converter outputs a grayscale current that is linear with respect to the grayscale data. Accordingly, a desired grayscale image is not displayed in a display panel, and image quality becomes poor.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and therefore, unless explicitly described to the contrary, it should not be taken as an acknowledgment or any form of suggestion that this information forms the prior art that is already known in this country to a person skilled in the art.